Design and Control of the Methoxy-Methyl-Heptane Process

Abstract

The chemical 2-methoxy-2-methylheptane (MMH) has been proposed as a gasoline additive to replace methyl tert-butyl ether (MTBE) in order to avoid groundwater contamination. The chemistry involves the liquid-phase reversible reaction of methanol with 2-methyl-1-heptene (MH) to form MMH. However, methanol and MH also undergo an undesirable reaction to form dimethyl ether (DME) and 2-methyl-2-heptanol (MHOH). The activation energies of the two competing reactions favor high reactor temperature, which is limited by catalyst activity to 400 K. Yield of the desired product is increased by either increasing reactor size or keeping the concentration of methanol in the reactor low by using a large excess of MH in the reactor. The latter strategy results in a large recycle flow rate of MH, which increases separation costs. Thus, this process demonstrates the classic design trade-off between reactor costs and separation costs. The purpose of this paper is to explore the economics and the dynamics of this process. The cost of the MH raw material and the values of the products are somewhat uncertain. Therefore the approach adopted in this paper is to design the process for a specified yield of MMH. For a given yield, there are optimum values of reactor size and recycle flow rate that minimize total annual cost. The flowsheet features a continuous stirred tank reactor, three distillation columns, and one recycle stream. A plantwide control structure is also developed that is capable of effectively handling large disturbances in production rate and operating parameters. The limiting reactant methanol is flow-controlled to set the production rate. The fresh feed of the other reactant MH is fed to maintain a fixed total flow rate of MH (fresh feed plus recycle).